Selective signal for telephone-circuits.



Patented Ma r. 6, I900. G. K. THOMPSON &. E. C. ROBES. SELECTIVE SIGNALFOR TELEPHONE CIRCUITS.

(Application filed July 18, 1899.) (No Model.)

THE norms PETERS co. vno'rouraa. WASHINGTON a c.

GEORGE K. THOMPSON, OF MALDEN, AND ERNEST O. ROBES, OF MEDFORD,

MASSACHUSETTS, ASSIGNORS TO THE AMERICAN BELL TELEPHONE COM- PANY, OFBOSTON, MASSACHUSETTS.

SELECTlVE SIGNAL FOR TELEPHONE-CIRCUITS.

SPECIFICATION forming part of Letters Patent No. 644,647, dated March 6,1900.

Application filed July 18, 1899. Serial No. 724,235. (No model.)

To all whom it may concern: ground branch and a polarized electromag- 50Be it known that we, GEORGE K. THOMPSON, netic call-bell includedtherein at each of two residing at Maiden, and ERNEST O. ROBES, reof thesubstations, or, in other words, the polarsiding at Medford, in thecounty of Middlesex ized call-bell of each station is in a normallyandState of Massachusetts, have invented detached earth branch, two of thesaid earth certain Improvements in Selective Signals for branches beingassociated with one main con- 5 5 Telephone-Circuits, of which thefollowing is ductor and the remaining two with the other,

a specification. and a relay-switch at each station connected Thisinvention relates to selective signals in series with a condenser in abridge between for telephone-circuits, and in particular to the mainconductors controls the connection that class wherein plus and minuscurrents of the ground branch and the appropriate may be transmitted atwill over either or both main conductor maintaining the normalsevconductors of a metallic circuit, the signalerance when the circuitis at rest, uniting the receiving devices of the several stations beingbranch to the main conductor while signals 15 connected in branches toearth from the said are being sent from the central station to any mainconductors, respectively. In organizasubstation and again severing thebranch tions of this class heretofore permanently-confromthe mainconductor after the signal has nected earth branches have exclusivelybeen duly been transmitted. employed, and it has therefore been imprac-The bridged relay at each substationis slug- 2o ticable for obviousreasons to arrange a fourgish or slow in demagnetization andpreferstation metallic circuit so fitted for operation ably has adelicately-balanced armature. It in connection with a common battery orcenis a neutral relaythat is to say, non-polartral-energy system oftransmitter and incomized-and is so constructed that when asuffiing-call-signal current-supply. ciently-strong intermittent currentof either 25 The object of the present invention is to direction iscaused to pass through its elecprovide a system of selective signalingfor astromagnet-coils its armature is attracted to sociation withfour-party metallic telephoneits forward position and remains attractedas circuits which, though involving the conneclong as the transmissionof such current contion of the signaling instruments at the sevtinues.

3o eral substations in earth or ground branches One of the polarizedbells of the two in the from one or the other of the main-circuitconbranches associated with each line conductor ductors, shall have allof the said ground is responsive by the vibration of its armature,branches primarily detached or disconnected and consequently by therepeated oscillations from their main-circuit conductors, respecof itsbell-hammer, to an intermittent current 3 5 tively,but so arrangedthattheyshall be fully of definite direction, while the armature ofavailable for use in connection with the said the bell in the otherbranch of the same main main conductors when a signal is to beselecconductor is in like manner adapted to retively transmitted to anysubstation. In such spond to a similar intermittent current, but asystem the main circuit normally and when of opposite polarity ordirection. By this 40 actually in use as a talking-circuit is mainmodeof connection it is evident that the two tained free from and-untrammeled by earth substation-bells of either main con'ductor can 0connections, and is therefore in a condition of be selectively operatedby sending intermitmuch higher efficiency than otherwise could tentcurrents of plus and minus direction, be the case. respectively, each intheir own time over 45 The system comprises a metallic main cirthe saidconductor, provided that the earth cuit extending between a centralstation and branches have first been united to their apfrom one to foursubstations so arranged that propriate main conductors by the action ofin the latter case each main conductor of the their associatedrelay-switches. circuit is provided with a normally-disunited At thecentral station the signaling-circuit is associated with a suitablesource of signaling-current and with means for connecting the same witheither main conductor and for transmitting therefrom an intermittentcurrent of either direction at will over the main conductor with whichat any moment it is connected. One pole of the said source is duringsuch transmission united to the said main conductor and the other isgrounded, and means are provided whereby a shunt-circuit of lowresistance is closed around the two bells associated with either mainconductor whenever a signaling-current is being sent over the other mainconductor for the operation of either of the bells thereof. Thuswhenever a bell at either of the four substations is operated by thepassage through its coils of an intermittent current of appropriatedirection the other bell associated with the same main conductor is notrung, because for its operation an opposed direction of current isrequired, and the two bells associated with the other main conductor arenot rung, forthe reason that though their earth branches also aretemporarily connected the electromagnets of both are shunted. Each belltherefore can at will be operated to the exclusion of the others.

In practice we prefer to employ a magnetoeleetric generator for therequisite source of current, the same being designed for the developmentof an alternating current and having one of its terminals grounded andthe other connected with an analyzing-commutator having two brushesunited to two supplymains, respectively, the said brushes being soplaced or adjusted that one of them receives for its supply-conductorthe plus or positive impulse only of each alternating wave developed bya complete revolution of the armature, while the other receives for itssupply-conductor the minus or negative impulse of the wave. By thismeans one of the said supply-conductors is made to receive anintermittent plus signaling-current and the other an intermittentminus-current. A set of signaling-keys is associated with thisgenerator, each key of the said set corresponding to and controlling aparticular and definite one of the substation-signals, two of the saidkeys being organized to connect one of the main conductors of thetelephone-circuit with the plus and minus current-supply conductors,respectively, and in either case to simultaneously unite an earthconnection to the other main conductor, and the remaining two keys beingadapted to reverse the process and to connect the supply-conductors,respectively, with the second main conductor and to ground the first.The attachment of the said ground connection to the main conductor notimmediately concerned in the signaling operation acts to establish thehereinbefore-mentioned shunt around the two callbells thereof.

Obviously a battery and reversing-keys might, if desired, be employed inlieu of the magneto-generator and selective-keys, if similarly arrangedfor the connection of the ground to the inactive main conductor.Moreover, itis evident that by suitable switching mechanism onecurrent-generator and one set of keys may be common to severalindependent telephone-circuits provided with independent earth branchesand call-bells.

In the drawings accompanying this specification, Figure 1 is a diagramof a telephone party-line circuit having four substations fitted withselective signals and associated with central-station manipulatingdevices exemplifying this invention. Fig. 2 is a side elevation of aform of sluggish intermittent current-relay suitable for employment inconnection with the said invention; and Fig. 3 shows a modification inthe arrangement of the analyzing-commutator of the signal-currentgenerator.

In Fig. 1, L is the main telephone-circuit, extending, as shown, betweenthe central station 0 and four substations S S S S It is a metallic ordouble conductor-circuit, and 2 and 3 represent the two conductorsthereof, respectively. J J represent spring-jacks or switch-socketswherewith it is provided in the switchboard at the central station, thenumber of such switch-sockets being dependent on the size of theswitchboard or, more generally, upon the magnitude of the centralstationand the number of lines connected therewith. P is a switch-plug at thecentral station adapted for insertion into the said sockets through theguide-ringsj thereof, and when so inserted its two conducting-surfaces pand 151 engage the corresponding socket connectionsj j respectively,thereby establishing union between the two main-circuit conductors 2 3and the cord-circuit conductors 10 and 12.

At each substation is the usual normallyopen telephone-bridge Iincluding, conductively, the secondary helix i of the transmitterinduction-coil I, and associated with a local circuit which, as shown,contains the transmitter-electrodes I, the telephone-receiver t and theprimary helix 1.' of said induction-coil. The receiver is shown as beingsupported on the hook of the usual telephone-switch c, which operates toclose the telephone-bridge and local circuit when the receiver is takenfrom the hook and to reopen both when it is replaced thereon, c beingthe switch-contact point for the bridge and c for the primary circuit.The said 10- cal circuit is in part formed of a portion 13 of a separatebridge D, presently to be described, and is thus made to include acondenser k, contained in the said separate bridge and having functionsin connection therewith. This arrangement of the substationtelephonebridge and local circuit is the one now generally adopted instandard common battery systems, and being familiar to persons skilledin the art needs no further remark, especially as it has no directconcern with the present invention.

At each substation in accordance with this invention the bridge D, towhich reference has been made,extends between the main conductors 2 3 ofthe telephone-circuit, say, at points z y. It contains a slowdischarging or demagnetizing neutral relay R, which is adapted torespond to the transmission over either conductor of the main circuit ofan intermittent signaling-current by the attraction of its armature r tothe front stop r thereof and by the steady maintenance of suchattraction as long as the transmission of such intermittent currentcontinues. Fig. 2 illustrates a form of relay which has been foundconvenient for this purpose and which operates satisfactorily. It isnon-polarized and consists of a single spool H, having an iron core withright-angle pole-pieces h 713, screwed to its two ends,respectively,thewhole being mounted on a base M by means of a bracket 'm, to which thecentral core and spool are secured by one of the pole-piece screws u.The armature r is Z-shaped and is delicately hung on an arbor h onpivots or in bearings of abrass plate h screwed to the pole-pieces, andof a similar plate placed immediately behind the first and similarlyattached to the inner surfaces of said polepieees, one end 0 of saidarmature being slightly heavier than the other to facilitate return tothe original position on the discontinuance of the vitalizing-current.The backstop r of the armature of this relay is supported in a smallbracket 0, mounted on one of the pole-pieces, and the bridge and branchconductor terminal springs may, as shown, be attached to the otherpole-piece. The bridge-wires 15 and 16 include between them the coil ofthe relay, and from the point 4 of conductor 15 a branch 17 in the formof a contact-spring extends toward the center of the relay and isprovided. near its end with a contact-point r within range of butnormally out of contact with a complementary point r carried by anotherspring g, which at its fixed end is united to a branch conductor o andat its free end bears lightly upon the armature at a point of theheavier arm 0 thereof quite close to the center 77, Thus when the relayis excited by the passage through its coils of an appropriate currentits armature *r swings into a position wherein its curved ends morenearly face the pole-pieces and in doing so slightly raises the spring qand establishes contact between the points 7 and r Such a relay has beenmade and operated under the terms of this invention. It

was wound with twelve thousand five hundred turns of No. 39 silk-coveredwire, giving a resistance of two thousand two hundred ohms and affordedgood results. It has been found that when a relay so constructed ,andadjusted is placed in series with a condenser and submitted to theaction of an intermittent current it will under a wide range of thefrequency of the intermissions attract its armature steadily to theposition wherein the end 0 is elevated and the points r r in contact andwill maintain such position notwithstanding the recurrence of anintermission between each two emissions of current as long as thetransmission of the said current continues. It is evident that thiscomes about by reason of the contemporaneous presence of severalcooperative conditions, among which may be mentioned the number of turnsand disposition of the winding, the quantity and arrangement of the ironin the core, polepieces, and armature, the presence of the condenser,and the mode of circuit connec tion. This invention is not, however,restricted to this or any other particular form of relay, and any relaycapable of maintaining the steady and constant attraction of itsarmature during the passage of an intermittent current through its coilsmay be employed in place thereof.

The relay containing bridge D extends from 3 on main conductor 2 to z onmain conductor 3 and, as already indicated, contains also the condenserin series with the relay. From a point 4 between point 3 and therelay-magnet on the relay-bridges D at two of the substations (in thisinstance S and 8) there extends a branch conductor 17 to the forwardcontact-stop r of the relay, and at the other two substations S and Sare similar connections, the point 4,from which the said branches start,bein g,however, there placed between the other main conductor 3 and thecondenser 70.

V at station S and V V and V at stations S S and 5, respectively, arenormallyopen or disconnected branches, each containing the polarizedcall-bell of its own station and constituted, as shown, of a portion 17,united to the line conductor, and in each station extending to the frontstop 4*, and aportion '12, extending from the earth or ground connectionE through the magnet-coils of the bell B, conductor 2%, and therelay-armature r, (or its equivalent spring q in Fig. 2,) carrying thecontact-point T The said earth branch V is normally discontinuousbetween the armature-point r and 'front stop r be cause the armature isnormally retracted; but when by the passage of an intermittent currentthrough the main conductor the relay R becomes excited and attracts itsarmature 'r the said two points are brought into contact and the earthbranch containing the polarized bell is closed to its own main conductorand can now be rung by the action of the said intermittent current if ofproper direction. The relay-armatures are therefore to be considered asswitches controlling the call-bell earth branches and adapted totemporarily unite them to the prearranged main conductors.

If the armatures of the bells B at the several substations were leftwithout bias or adjustment, they would be adapted to operate vigorouslyin response to alternating currents; but as the currents to which theyare desired to respond are intermittent currents and as of the twostations associated conductively when their earth branches are closedwith each line one is required to respond to intermittent currents ofplus and the other to intermittent currents of minus sign it isnecessary to so adjust or bias the bell-magnets or their armatures thatneither shall be able to respond and ring its bell with a current ofsign appropriated to the other. Accordingly it is assumed that the bellB at station S is to be operated only by an intermittent current ofplus-sign or positive direction transmitted from the central station 0over the metalliccircuit main conductor 2, and in order that this may bedone the spring b is arranged to press upon one end of the armature b ofthe said bell and to hold it in the position to which, if left free, itwould be attracted by currents of minus-sign. Vhen, therefore, anintermittent current of minus-sign is transmitted over the said mainconductor 2, the said armature at station S will remain quiescent,because although its relay-armature is attracted and the earth branch Vcontaining its actuating-magnet thereby closed, a minus-current impulsecan do no more than hold it where it is; but a plus-current impulse willattract the other end of the armature against the force of the biasingspring rocking it on its center and causing its hammer to forciblystrike the gongs 19 Moreover, a succession of such plus impulses willproduce a succession of such oscillations and the bell will be rungcontinuously until the application of the current to the main lineceases. The bell at station S though also in a branch V from the samemain conductor 2, is oppositely biased by its armaturespring If. Ittherefore refuses to respond to plus-current impulses, and requires forits operation the transmission of an intermittent current of minus-signor negative direction over the same main-line conductor 2. In likemanner the armatures of the bell-1n agnets of stations S and S areprovided with biasing springs adjusted oppositely, and these bells arethereby adapted to respond, respectively, to plus and minusintermittentcurrents transmitted over the main-line conductor 3.

G at the central station represents a suitable source of currentdesigned to develop and to deliver to the supply-conductors Q andintermittent currents of positive and negative direction, respectively,the said conductors being both in dilferent positions of thegenerating-armature, united to one terminal of the winding thereof bymeans of their brush-springs 00 01 which press upon theanalyzing-commutator e. The said commutator is made with anon-conducting portion e and a conducting portion e each portion passingunder the brushes 0: :0 once in each revolution. The armature-windinghas one terminal attached to the said conducting portion and the otherto the metal project ing piece a, on which the brush-spring to which thepermanent ground connection 21 is attached, continuously presses. Thebrushes 0: .12 are so adjusted that the former is in connection with thecommutator-conductor during the development of the positive and thelatter during the development of the negative impulse of the generator.

K K K K are signaling-keys placed in front of the operator, and by aswitch, as 8, they may be switched to any plug P to ring any bell of thecircuit L within whose switchsocket the plug is placed. They maybeoperated by the buttons X X X X, and each corresponds with and controlsthe bell of one station. The springs of the said keys are all marked 25and 27 and connect with the keyboard and plug-circuit wires 10 and 12,and thereby \viththe main conductors 2 and 3, respectively. To make thekeyboard-circuit continuous and at the same time to give each key theproper control over it, the springs 25 and 27 of the keys normally presson the contact-points 26 and 28, respectively, except that spring 27 ofkey K presses normally on an earthcontact 22 and that the key K whichhas but one active spring 25,(since the keyboard-circuit does not needto continue farther,) has no normal connection with a contact-point. Theearth connection E branches into the three first keys, passing in eachto one of the operative contact-points 22, and in key K as beforestated, it passes likewise to the resting-contact to furnish the returnground connection for key K. The positive supplyconductorQbranchesintokeys K and K and the negative supply-conductor W into keys K K Ifthe plug P be placed in either switchsocket and the switch 5 be properlyplaced, the bell B at substation S will be rung by pressing the button Xof key K. This operation separates the springs 25 27, representing themain conductors, from the points 26 28, thus disconnecting the keyboardextension-circuits, bringing spring 25 into contact with theground-connection point 22 and spring 27 into contact with the terminalpoint 23 of the positive-impulse-supply conductor Q. Main conductor2 isthus connected with a dead-ground or an earth connection whoseresistance is negligible at the central station and main conductor 3 isconnected with the supply-conductor Q. Consequently an intermittent pluscurrent is transmitted over main conductor 3 and the main conductor 2furnishes a return therefor, having an electrostatic connectiontherewith through the condensers 76 in the several station-bridges. Ametallic electrostatic circuit for the relays R at all stations is thuscompleted, and. the intermittent current pulsating therein actuates allof the said relays, which thereupon attract their armatures, and thusbring the station earth branches into direct conductive connection withthe main wires 2 and 3, re-

eratejthe bell B at station S is alone operated, the keyK havingselected it to the exclusion of the others. The intermittent plusimpulses thrown on conductor 3 pass to the earth branch at station S andthrough the magnet of bell B to earth E. This bell being biased torespond by the oscillation or vibration of its armature to broken pluscurrents rings continuously as long as the key-pressure is sustained;but the bell at station S does not ring, because, though the currentpasses through its magnet-coils, its effect is such as a to reinforcethe biasing-spring b the bell at this station being adjusted or biasedto respond to broken negative impulses only, and the bells B at stationsS and S do not ring, because by the connection with the earthestablished in the key K between the spring 25 and the point 22 a shuntof extremely-low resistance has been formed round the high-resistancemagnets of the bells B at the said stations, for the intermittentcurrents or current impulses developed in the return portion of themetallic main circuit L through the condensers 71: and at stations S andS on reaching the junction-points 4 have two paths before themone ofhigh resistance to earth through the station bell-magnet and the otherof low resistance to earth at the central station by way of main wire 2,socketspring 7' plug-conductor p, switch-cord conductor 10, point 22,and ground-wire E The bells B at the stations S S are thusshortcircuited, and for this reason they also remain inoperative. Whenthe key K is operated, minus impulses are transmitted over mainconductor 3 and the bell at station S rings, the bell at station S beingresponsive to plus currents only and those at stations S and S beingshunted as before, and in the same manner the keys K and K will, whenoperated, cause the bells at stations S and S to selectively ring bytransmitting plus and minus currents over the main conductor 2, thecurrent adapted for either being of direction opposed to the operationof the other and the shunt in both cases being established about thebell-magnets at stations S and S The discharge of the condenser 70 takesplace in the arrangement so far described, through the electromagnet ofthe bell B. It necessarily, therefore, is not as strong as the chargingimpulse which reaches the condenser directly from the main line. Whilethe arrangement under these conditions works satisfactorily, there is awide range of individuality in bell-magnets, and to insure successfuloperation under all conceivable conditions we have devised and arrangedmeans for grounding the line over which the intermittent current isbeing transmitted on and during the intermissions of said current, sothat a circuit of low resistance is provided for the discharge impulsefrom the condenser through the relay. By this means the impulse is muchstronger and the retention of magnetization more complete. The saidmeans is illustrated by Fig. 3. It comprehends two back contact-stops 0000 for the brushes m 00 respectively, which form the source terminals ofthe supply-conductors Q and l/V, the formation of the non-conductingportion a of the analyzing-commutator e, with a larger radius than thatof the conducting portion e and a connection from earth E to bothback-stops by the conductor 22. Thus the line conductor, which at anytime through the intermediation of one of the supply-conductors and theoperation of the appropriate key receives the intermittent currentimpulses from the generator G while the conducting portion 6 of thecommutator is passing under the brush of the said supply-com ductor, isgrounded during the intermissions of said current, when the largernon-conducting portion a passing under the same brush presses it againstthe back-stop r or w, as the case may be.

Having now fully set forth the structure, arrangement, and the mode inwhich the invention is operated, we claim- 1. In a telephone signalingsystem and apparatus, the combination with a metallic ordouble-conductor main circuit; and means for transmitting anintermittent current of definite direction over one of the conductorsthereof; of a bridge of said circuit containing a condenser, and a relayresponsive to such a current and adapted to maintain the steadyattraction of its armature during the passage thereof; anormally-disconnected earth branch controlled by the said armature, andunited during the attracted position thereof to the said main-circuitconductor; and a po larized bell having its magnet in the said earthbranch, and having its armature biased to move in one direction witheach emission, and oppositely with each intermission of the saidcurrent.

2. The combination in a telephone-signaling system, of a metallic ordouble-conductor main circuit; and a source of intermittent current ofdefinite direction connecting with one of the conductors thereof; with asub scribers signaling apparatus comprising a bridge between the mainconductors of said circuit; a condenser, and a relay in the said bridge,the latter organized to attract its armature and to steadily maintainthe attraction thereof during the passage over the said main conductorof the said intermittent current; a ground branch of the said mainconductor normally disconnected therefrom, but brought into directconnection therewith by the action of the said relay-armature whenattracted; and a polarized bell having its magnet in said branch, andits armature biased or adjusted to vibrate in response to the saidmain-line current of single direction; whereby the temporary connectionof the ground branch, and a continuous ringing of the polarized bellincluded therein, may both be effected by the same intermittent currentof the said source, acting through the steadily-attractedrelay-armature, and the interl station for transmitting an intermittentcurn1ittently-attracted bell-armature; substantially as set forth.

3. lhe combination of a metallic telephonecircuit extending between acentral station and two substations; a ground branch at each substationextending from one of the maincircuit conductors, but normally detachedtherefrom; a sluggish or slow-acting neutral switching-relay asindicated, at each substation, bridgedin series with a condenser betweenthe main conductors of said circuit, and adapted on the excitement ofits magnet to switch the said ground branch into connection with theappropriate main conductor; a polarized bell, biased or adjusted torespond to intermittent currents of definite direction in one, and asimilar bell oppositely polarized or oppositely biased to respond tolike intermittent currents of opposite direction in the other of saidground branches; and means at the central station for impressing uponthe said appropriate main cond uctor,intermittent currents of eitherdirection for the selective operation of either bell.

4. In a telephone system, a metallic main circuit extending between acentral station and two substations; a slow-acting neutralswitching-relay responsive by the protracted attraction of its armatureto the passage through its coils of an intermittent current, connectedserially with a condenser in a bridge at each substation between the twoconductors of said main circuit; normally-discontinuous earth branches,one at each substation, extending from the said main-circuit conductorsrespectively, and each including a polarized electromagnetic bellresponsive by the vibration of its armature to an intermittent current,the said earth branches being each adapted for connection with its ownmain conductorby the operation of the switching-relay at the samestation; and means at the central station for transmitting anintermittent current over either main conductor of said metalliccircuit, and for simultaneously grounding the other main conductor;substantially as and for the purpose set forth.

5. A system of selective signaling, comprising a main metallictelephone-circuit extending to four substations; a bridge at eachsubstation containing a slow acting neutral switching-relay,andacondenser; a normally detached and discontinuous earth branch at eachsubstation, two of the said branches being associated with one, and twowith the other of the main conductors of said metallic circuit, and alladapted to be switched into direct connection with their respective mainconductors by the action of their respective relays; a polarized bell ineach earth branch, those associated with the same main conductor beingbiased or adjusted oppositely, and thereby made respectively responsiveto intermittent currents of opposite direction respectively transmittedover the said conductor; and means as indicated at the central rent ofeither direction over either main conductor, and for simultaneouslyestablishing a shunt of relatively-low resistance around thebell-magnets associated with the other main conductor; whereby in acircuit having no permanentlyconnected earth branches, eithersubstation-bell may be selectively operated, to the exclusion of theothers.

6. The combination of the two main conductors of a metallictelephone-circuit having four substations; with twonormally-disconnected ground branches located at two of said substationsand adapted for direct connection with one of the said main conductors,each branch containing an oppositely-polarized or oppositely-biasedelectromagnetic call-bell; two other ground branches similarly detachedfrom but adapted for connection with the other main conductor, andrespectively containing the oppositely-biased bells of the otherstations; a normally-open shunt, of resistance low relatively to that ofthe bell-magnets, around each such bell-magnet, means for uniting all ofthe said earth branches to their respective main conductors when acurrent adapted for the operation of any of the bells is transmittedover either main conductor; and means at the central station for closingthe shunt-circuit of the two bells of either main conductor, when anintermittent current adapted for the operation of either of the twobells of the other main conductor is transmitted over the said othermain conductor; substantially as described.

7. In a system of selective signals for a telephone-circuit, thecombination of a normallygroundless do uble-cond uctor main circuitextending between a central station and four substations; a normallydetached earth branch at each substation, two of them being associatedWith one conductor, and two with the other conduetorof said maincircuit; a slow-acting neutral switching-relay at each substation,bridged in series with a condenser between the conductors of said maincircuit, all of the said relays being responsive to intermittentcurrents of either direction over either main conductor, and adaptedwhen excited thereby to attract and maintain the attraction of theirarmatures, which constitute switches to temporarily unite the earthbranches at their respective stations to the prearranged mainconductors; a polarized electromagnetic bell connected in the earthbranch of earth substation, those in the two branches of either mainconductor being oppositely biased or adjusted to respond to intermittentcurrents of opposite direction respectively; a source ofsignaling-current at the central station having one pole grounded, andorganized to deliver intermittent currents of plus and min us directionrespectively to different terminal conductors; an independent earthconnection at said central station; and a signaling-key at the saidcentral station for each substation-bell,adapted when operated toconnect the main conductor'thereof with the appropriate source terminal,and the other main conductor with the said independent earth connection;whereby an intermittent current of definite direction may be transmittedover a particular main conductor for the selective operation of thecorresponding substation-bell, and of the relays at all stations, thetwo bells of the other main conductor being shunted by the saidindependent earth connection.

8. In a telephone signaling system and apparatus, the combination witheither main conductor of a metallic telephone-line circuit; of anormally-disconnected earth branch at a substation; a polarized bellbiased to respond to an intermittent current of definite direction andhaving its electromagnet included in said branch; a bridge of the maincircuit at said station including a condenser,

two subscribing witnesses, this 13th day of July, 1899.

GEORGE K. THOMPSON. ERNEST C. ROBES.

Witnesses:

GEO. WILLIS PIERCE, FRANK C. LooKWooD.

ion in Letters Patent No, 644,647"

Correct It is hereby certified that in Letters Patent No. 644,647,granted March 6, 1900, upon the application of George K. Thompson, ofMalden, and Ernest O. Robes, ofMedford,- Massachusetts, for animprovement in Selection Signals for Telephone-Circuits, an errorappears in the printed specification requiring correction, as follows:In line 123, page 6 the Word earth should read each,- and that the saidLetters Patent should be read with this correction therein that the samemay conform to the record of the case in the Patent Office.

Signed, countersigned, and sealed this 20th day of March, A. D., 1900.

[SEAL] THOS. RYAN,

First Assistant Secretary of the Interior. Oountersigned O. H. DUELL,

Commissioner of Patents.

